How Wilhelm Roentgen Discovered X-rays

So, why is an x-ray called an x-ray?  The short answer is that when Wilhelm Roentgen discovered x-rays in 1895, he called them x-rays outright because x stood for unknown and he knew he had found a new, previously unknown ray.  Ok, bye!  But I still haven’t answered how did Roentgen know he had discovered a new kind of light and why did the name stick outside of Germany and why do x-rays have a totally different name in German?  I’ll tell you. 

Table of Contents

Crookes’ Study

Lenard’s Study

Roentgen’s Discovery: The X-ray

The Different Name of X-ray in Germany


Crookes’ Study

I would like to start in the 1880s, when an English chemist named William Crookes sent some of his photographic plates back for a refund as he said that they were defective.  What Crookes didn’t realize was that the plates were fine, he had accidentally developed the film himself with x-rays! 

See, Crookes had been studying something called cathode rays and had proved that they could melt a piece of platinum if the rays were focused on the metal.  What Crookes didn’t realize until many years later, however, was that when the cathode rays hit the platinum, it produces x-rays which can develop film even if the film is protected.  As Crookes was focused on the melting platinum and not the photographs, he just got his refund and moved on. 

Fast forward to 1892, when a 35-year-old Heinrich Hertz, already famous for discovering radio waves 4 years before, decided to recreate Crookes experiment of melting metals with cathode rays (Hertz didn’t know about the photographic plates).  Hertz wished to see if an unfocused beam could melt an even thinner piece of metal so he put a super-thin gold leaf on the back of a tube.

To his shock, the cathode rays seemed to just go straight through the gold to make the glass behind it glow!  Hertz then found that cathode rays could travel through all metals as long as they were thin enough.  Hertz usually liked to do all important experiments himself but he was starting to get a lot of headaches so he suggested that his assistant, Phillip Lenard, should experiment on this phenomenon.

Lenard’s Study

Lenard then tried something radical.  Cathode rays can only be made in a tube with a high vacuum, but Lenard wondered if he could use a metal “window” to move the rays into the room.  Lenard then took a cathode ray tube with a small hole, covered the hole with aluminum foil, and sprinkled grains of fluorescents on the outside, as Lenard knew that cathode rays, like UV rays, are invisible to the naked eye but can make fluorescents glow. 

When he turned on his tube, the phosphors glowed *outside* the tube!  Even more startingly, the fluorescents continued to glow a few inches away from his “window”. This experiment, like Crookes’ experiment, produced x-rays and unlike Crookes’s experiment, Lenard was covering his tube and examining it with phosphors outside of the tube. 

However, Lenard was using fluorescents made of light elements with low atomic numbers that was transparent to x-rays and, therefore, he just missed discovering the x-ray. Tragically, just as Lenard was publishing his results, Hertz died on January 1, 1894 at the age of 36 from blood poisoning. 

Lenard basically stopped his research to focus on editing a collection of Hertz’s work, and it then took him two years to find a stable position at a university to continue his research, which is almost exactly when he learned that 50-year-old Wilhelm Roentgen had used “his” Lenard tube with a different fluorescent to discover the x-ray[1]!

Roentgen’s Discovery: The X-ray

Wilhelm Roentgen had been interested in Hertz’s and Lenard’s results for many years and began to study cathode rays in 1894.  He even wrote to Lenard for some advice on how to build “his” tube with a window.  However, Roentgen only had time to seriously study cathode rays in October of 1895.

After only a few weeks of study, on November 8th, 1895, Roentgen was working late and he noticed that a fluorescent paper on the side of the covered tube started to glow. The fact that the fluorescent glowed on the side of the tube completely shocked Roentgen as he recalled in his one and only magazine interview given in April of 1896, “the shield which covered (the sides of the tube) was impervious to any light known”. 

Roentgen then found that the glowing on his fluorescent was far brighter on the other side of the aluminum “window” where it was clearly emanating.  Roentgen started holding up everything he could find to see if it could block the “light”, a book, a deck of cards, tin foil, pine wood, aluminum, rubber, glass, and he even tried a glass of water! 

While Rontgen was basically throwing everything he could find between the tube and the fluorescent screen, he tried something completely natural and simultaneously earth-shattering, he held up a small lead disk with his hand.  As he waited for the image of a shadow of a disk to appear on the screen, he saw something beyond belief: the shadows from the bones in his own hand! 

No wonder he told his wife, Bertha, that he was working on something that would probably make people say “Roentgen has surely gone crazy[2].”  Rontgen was instantly convinced that he was dealing with a new kind of light that was vastly different than the “cathode ray” that produced it and more powerful than anything ever seen. 

Roentgen recalled, “It seemed at first a new kind of invisible light.  It was clearly something new, something unrecorded.”  As this was a new ray of “light” Rontgen decided to name “This new kind of ray” an x-ray “for brevity’s sake …and to distinguish them from others [rays].”  X being the mark of unknown in Algebra. 

Roentgen knew his discovery seemed fantastical and worried that no one would believe him, however, he was also an amateur photographer and found to his relief that these new rays could develop film while they were still in their protective coating (just like Crookes found). 

It was for that reason on December 22, 1895, Roentgen asked his wife Bertha if she would come to the laboratory and hold her hand on a photographic plate for 15 minutes.  They created a ghostly photo that showed her bones and the large metal ring on her finger.  When Bertha saw this image, the first photograph of bones of inside an intact hand, she supposedly exclaimed, “I have seen my own death![3]” 

Rontgen published on December 28th, 1895, and within a week, according to Roentgen “all hell broke loose”.  Almost everyone wanted to play with x-rays (including most scientists) and soon there were traveling x-ray exhibitions, poetry, art, and prudish controversies. 

Rontgen wanted nothing to do with fame but was convinced to give a talk by the Kaiser on January 23rd to a packed house.  After Roentgen took another x-ray of an attendee’s hand, the room erupted in cheers and everyone decided to rename x-rays “Roentgen rays” which they are called in Germany to this very day.

The Different Name of X-ray in Germany

At first, in America, England and France, scientists and the general public used both x-rays and Roentgen rays interchangeably.  Like this terrible poem titled “x-actly so” which mentions both “x-rays’ ways” and “these naughty, naughty, Roentgen Rays”.  However, as modern purveyors of “adult films” know all too well, the “x” is a very attractive image for advertisers: it is used as “x” marks the spot, so it is comforting to people while being almost unused in the beginning of words so it is distinctive and attracts the eye. 

Therefore, the advertisers selling x-ray sunglasses and x-ray proof underwear and x-ray stove polish vastly preferred the term x-ray to a foreign-sounding Roentgen Rays and within a few years x-rays were the universal name outside of Germany. As you can see from this fabulous silent film called “The X-ray Fiend” from 1897 where they never mention Roentgen.  In Germany, of course, not only was the name Roentgen not foreign, but using it was a way of having German pride in its discovery.

And that is how x-rays got their name outside of Germany, and “der Roentgenstahl” got that name in Germany!  The discovery of x-rays did more than just scandalize Victorians with the sight of “naked” bones, it also led to a rash of scientific discoveries including radioactive decay!  And, how radioactive decay got its name is next time on the lightning tamers!


[1] According to Howard Seliger Hertz died in January 1894 and “by January 1896…[Lenard] finally returned to his cathode rays” found in Seliger, Howard “Wilhelm Conrad Röntgen and the Glimmer of Light” Physics Today Nov. 1995 p. 26

[2] referenced on p. 67 Glasser, Otto Wilhelm Conrad Roentgen and the Early History of the Roentgen Ray 1933,

[3] p 95 “The Human Body” H Newquist

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